pv: pv_get_method fixed error "pv_get_method(): no CSEQ header"

GH kamailio#2761: Error obeserver when sent HTTP reply like

HTTP/1.1 200 OK
Sia: SIP/2.0/TCP 8.8.8.8:39813
Content-Type: application/json
Server: kamailio
Content-Length: 49

{"data":{"status-code":200,"reason-phrase":"OK"}}

src/modules/pv/pv_core.c

@@ -147,6 +147,9 @@ int pv_get_method(struct sip_msg *msg, pv_param_t *param,
 				(int)msg->first_line.u.request.method_value);
 	}
 
+	if (IS_HTTP_REPLY(msg))
+		return pv_get_null(msg, param, res);
+
 	if(msg->cseq==NULL && ((parse_headers(msg, HDR_CSEQ_F, 0)==-1) ||
 				(msg->cseq==NULL)))
 	{